1. Field of the Invention
The present invention relates to a method for monitoring an automation system with a control unit and a plurality of terminals. In particular, the present invention relates to the reliable detection of a plurality of signal changes in a monitoring message between a terminal and a control unit in an automation system by monitoring and determining preceding states of one or more terminals of an automation system.
2. Description of Related Art
In automation systems with a plurality of terminals (also referred to as programmable controllers) and control units (also referred to as control and monitoring stations or operator stations) events are mapped to binary signals. The terminals detect signal changes (level-triggered changes) and use a monitoring message to report them to one or more control units. The control unit processes the reported signal level.
In the prior art, when a first message reporting a signal change is sent from the terminal, a second message cannot be sent until the control unit has confirmed that it has received the first message. The contents of the second message, for example, can indicate that the signal has changed again. The control unit confirms the receipt of a message by sending an acknowledgment message, for example.
If a single signal is reported in a monitoring message, the last two signal changes may be detected in the prior art systems. The last two signal changes may be detected by:
a) providing additional “overflow” information in the monitoring message indicating that one or more signal changes could not be reported; b) receiving a message with the identical signal state as the signal, stored in the control unit, indicating the last current state detected by the terminal.
To increase the capacity of the event log and to use system resources more effectively, monitoring messages which contain more than one signal, e.g., eight signals, are used. The additional information in the form of overflow flags in the monitoring message and the fact that a message has been received represents a 1:n relation if there is a number of signals. An allocation of the additional information in the overflow flag and of the event “receipt of a monitoring message” to a single signal or several signals is no longer possible. It is therefore useless for general signal tracing. The information available in the monitoring message is reduced instead to the respective signal state; a change in the meantime from the initial to a second state and back again is not detected.
In other words, conventional techniques cannot provide the control unit with accurate state changes of a signal detected by a terminal. The control unit cannot receive accurate state changes of the signal at least in part because of the undetected state changes that occur during the transmission of the monitoring message from the terminal to the control unit. That is, when more than one state change of the signal occurs during the transmission of the monitoring message, these intermediate state changes are not reported to the control unit. Instead, these intermediate state changes are unnoticed by the control unit.